Classifications of Pumps

Selecting between Centrifugal Pumps and Positive Displacement Pumps

Pumps are in general classified as Centrifugal Pumps (or Roto-dynamic pumps) and Positive Displacement Pumps.

Centrifugal Pumps (Roto-dynamic pumps)

a

The centrifugal or roto-dynamic pump produce a head and a flow by increasing the velocity of the liquid through the machine with the help of a rotating vane impeller. Centrifugal pumps include radial, axial and mixed flow units.

Centrifugal pumps can further be classified as

end suction pumps in-line pumps double suction pumps vertical multistage pumps horizontal multistage pumps submersible pumps self-priming pumps axial-flow pumps regenerative pumps

Positive Displacement Pumps

The positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. The positive displacement pump delivers a constant volume of liquid for each cycle against varying discharge pressure or head.

The positive displacement pump can be classified as:

Reciprocating pumps - piston, plunger and diaphragm Power pumps Steam pumps Rotary pumps - gear, lobe, screw, vane, regenerative (peripheral) and progressive cavity

Selecting between Centrifugal or Positive Displacement Pumps

Selecting between a Centrifugal Pump or a Positive Displacement Pump is not always straight forward.

Flow Rate and Pressure Head

The two types of pumps behave very differently regarding pressure head and flow rate:

The Centrifugal Pump has varying flow depending on the system pressure or head The Positive Displacement Pump has more or less a constant flow regardless of the system pressure or head. Positive Displacement

pumps generally gives more pressure than Centrifugal Pump's.

Capacity and Viscosity

Another major difference between the pump types is the effect of viscosity on the capacity:

In the Centrifugal Pump the flow is reduced when the viscosity is increased In the Positive Displacement Pump the flow is increased when viscosity is increased

Liquids with high viscosity fills the clearances of a Positive Displacement Pump causing a higher volumetric efficiency and a Positive Displacement Pump is better suited for high viscosity applications. A Centrifugal Pump becomes very inefficient at even modest viscosity.

Mechanical Efficiency

The pumps behaves different considering mechanical efficiency as well.

Changing the system pressure or head has little or no effect on the flow rate in the Positive Displacement Pump Changing the system pressure or head has a dramatic effect on the flow rate in the Centrifugal Pump

Net Positive Suction Head - NPSH

Another consideration is the Net Positive Suction Head NPSH.

In a Centrifugal Pump, NPSH varies as a function of flow determined by pressure In a Positive Displacement Pump, NPSH varies as a function of flow determined by speed. Reducing the speed of the Positive

Displacement Pump pump, reduces the NPSH

NPSH - Net Positive Suction HeadA definition and an introduction to Net Positive Suction Head - NPSH

Low pressure at the suction side of a pump can encounter the fluid to start boiling with

reduced efficiency

cavitation

damage

of the pump as a result. Boiling starts when the pressure in the liquid is reduced to the vapor pressure of the fluid at the actual temperature.

To characterize the potential for boiling and cavitation, the difference between the total head on the suction side of the pump - close to the impeller, and the liquid vapor pressure at the actual temperature, can be used.

Suction Head

Based on the Energy Equation - the suction head in the fluid close to the impeller can be expressed as the sum of the static and the velocity head:

hs = ps / γ + vs2 / 2 g         (1)

where

hs = suction head close to the impeller

ps = static pressure in the fluid close to the impeller

γ = specific weight of the fluid

vs = velocity of fluid

g = acceleration of gravity

Liquids Vapor Head

The liquids vapor head at the actual temperature can be expressed as:

hv = pv / γ         (2)

where

hv = vapor head

pv = vapor pressure

Note! The vapor pressure in fluids depends on temperature. Water, our most common fluid, starts boiling at 20 oCif the absolute pressure in the fluid is 2.3 kN/m2. For an absolute pressure of 47.5 kN/m2, the water starts boiling at 80 oC. At an absolute pressure of 101.3 kN/m2 (normal atmosphere), the boiling starts at 100 oC. 

Net Positive Suction Head - NPSH

The Net Positive Suction Head - NPSH - can be expressed as the difference between the Suction Head and the Liquids Vapor Head and expressed like

NPSH = hs - hv         (3)

or, by combining (1) and (2)

NPSH = ps / γ + vs2 / 2 g - pv / γ         (3b)

Available NPSH - NPSHa or NPSHA

The Net Positive Suction Head made available the suction system for the pump is often named NPSHa. The NPSHacan be determined during design and construction, or determined experimentally from the actual physical system.

The available NPSHa can be calculated with the Energy Equation. For a common application - where the pump lifts a fluid from an open tank at one level to an other, the energy or head at the surface of the tank is the same as the energy or head before the pump impeller and can be expressed as:

h0 = hs + hl         (4)

where

h0 = head at surface

hs = head before the impeller

hl = head loss from the surface to impeller - major and minor loss in the suction pipe

In an open tank the head at surface can be expressed as:

h0 = p0 / γ = patm / γ         (4b)

For a closed pressurized tank the absolute static pressure inside the tank must be used.

The head before the impeller can be expressed as:

hs = ps / γ + vs2 / 2 g + he         (4c)

where

he = elevation from surface to pump - positive if pump is above the tank, negative if the pump is below the tank

Transforming (4) with (4b) and (4c):

patm / γ = ps / γ + vs2 / 2 g + he + hl         (4d)

The head available before the impeller can be expressed as:

ps / γ + vs2 / 2 g = patm / γ - he - hl         (4e)

or as the available NPSHa:

NPSHa = patm / γ - he - hl - pv / γ         (4f)

Available NPSHa - the Pump is above the Tank

If the pump is positioned above the tank, the elevation - he - is positive and the NPSHa decreases when the elevation of the pump increases.

At some level the NPSHa will be reduced to zero and the fluid starts to evaporate.

Available NPSHa - the Pump is below the Tank

If the pump is positioned below the tank, the elevation - he - is negative and the NPSHa increases when the elevation of the pump decreases (lowering the pump).

It's always possible to increase the NPSHa by lowering the pump (as long as the major and minor head loss due to a longer pipe don't increase it more). This is important and it is common to lower the pump when pumping fluids close to evaporation temperature.

Required NPSH - NPSHr or NPSHR

The NPSHr, called as the Net Suction Head as required by the pump in order to prevent cavitation for safe and reliable operation of the pump.

The required NPSHr for a particular pump is in general determined experimentally by the pump manufacturerand a part of the documentation of the pump.

The available NPSHa of the system should always exceeded the required NPSHr of the pump to avoid vaporization and cavitation of the impellers eye. The available NPSHa should in general be significant higher than the required NPSHr to avoid that head loss in the suction pipe and in the pump casing, local velocity accelerations and pressure decreases, start boiling the fluid on the impeller surface.

Note that the required NPSHr increases with the square capacity.

Pumps with double-suction impellers has lower NPSHr than pumps with single-suction impellers. A pump with a double-suction impeller is considered hydraulically balanced but is susceptible to an uneven flow on both sides with improper pipe-work.

Example - Pumping Water from an Open Tank

When increasing the the elevation for a pump located above a tank, the fluid will start to evaporate at a maximum level for the actual temperature.

At the maximum elevation NPSHa is zero. The maximum elevation can therefore be expressed by (4f):

NPSHa = patm / γ - he - hl - pv / γ = 0

For optimal theoretical conditions we neglect the major and minor head loss. The elevation head can then be expressed as:

he = patm / γ - pv / γ         (5)

The maximum elevation or suction head for an open tank depends on the atmospheric pressure - which in general can be regarded as constant, and the vapor pressure of the fluid - which in general vary with temperature, especially for water.

The absolute vapor pressure of water at temperature 20 oC is 2.3 kN/m2. The maximum theoretical elevation height is therefore:

he = (101.33 kN/m2) / (9.80 kN/m3) - (2.3 kN/m2) / (9.80 kN/m3)

    = 10.1 m

Due to the head loss in the suction pipe and the local conditions inside the pump - the theoretical maximum elevation is significantly decreased.

The maximum theoretical elevation of a pump above an open water tank at different temperatures can be found from the table below.

Suction Head as Affected by Temperature

TemperatureVapor

PressureMax. elevation

(oC) (oF) (kN/m2) (m) (ft)

0 32 0.6 10.3 33.8

5 41 0.9 10.2 33.5

10 50 1.2 10.2 33.5

15 59 1.7 10.2 33.5

20 68 2.3 10.1 33.1

25 77 3.2 10.0 32.8

30 86 4.3 9.9 32.5

35 95 5.6 9.8 32.2

40 104 7.7 9.5 31.2

45 113 9.6 9.4 30.8

50 122 12.5 9.1 29.9

55 131 15.7 8.7 28.5

60 140 20 8.3 27.2

65 149 25 7.8 25.6

70 158 32.1 7.1 23.3

75 167 38.6 6.4 21

80 176 47.5 5.5 18

85 185 57.8 4.4 14.4

90 194 70 3.2 10.5

TemperatureVapor

PressureMax. elevation

(oC) (oF) (kN/m2) (m) (ft)

95 203 84.5 1.7 5.6

100 212 101.33 0.0 0

Pumping Hydrocarbons

Be aware that the NPSH specification provided by the manufacturer in general is for use with cold water. For hydrocarbons these values must be lowered to account for the vapor release properties of complex organic liquids.

FluidTemperatur

e (oC)

Vapor Pressure

(kPa abs)

Ethanol20 5.9

65 58.2

Methyl Acetate

20 22.8

55 93.9

Note that the head developed by a pump is independent of the liquid, and that the performance curves for water from the manufacturer can be used for Newtonian liquids like gasoline, diesel or similar. Be aware that required power depends on liquid density and must be adjusted.